Internal medical examinations are typically carried out by using an endoscope in which the eye or a CCD camera images the view relayed from the distal end of a shaft of the probe. In a flexible endoscope, the image may be relayed using a coherent fiber bundle containing thousands of individual fibers; in a rigid probe or borescope, the image may be relayed via a system of lenses or rods. Effectively this gives a view of the surface of the relevant medical target, but to see changes in the structure below the surface, it is desirable to be able to obtain a cross-sectional image from within the bulk of the tissue. This is the capability which OCT can provide. Variants of OCT have been described which can extract additional information, such as blood flow velocity (Doppler), or alignment of muscle fiber (polarization).
OCT may be used in the visible part of the spectrum for retinal examination, but to obtain reasonable penetration depth in other, more strongly scattering, tissues it is necessary to move to infrared wavelengths.
OCT is based on the use of interferometry, where light in the measurement arm of an interferometer is passed to the object to be examined and a portion is scattered back to the interferometer. Light in the reference arm is passed to a mirror at a known distance and a reference beam is reflected back. The scattered measurement beam and the reflected reference beam are combined, and the interference between these two beams is detected and used to provide data about the examined object.
Thus optical coherence tomography uses interferometry and the coherence properties of light to obtain depth-resolved images within a scattering medium, providing penetration and resolution which cannot be achieved with confocal microscopy alone. Clinically useful cross-sectional images of the retina and epithelial tissues have been obtained to a depth of 2-3 mm.
There are three main types of OCT which can be categorized as follows:
Time domain OCT; this uses a low coherence source and scans axially (in depth) by altering the reference path length of the interferometer.
Spectral domain OCT; this uses a wide spectrum (i.e., low coherence) source, a stationary interferometer and a spectrometer. The spectrum of the interferogram is examined by the spectrometer and the axial response is obtained as the Fourier transform of the spectrum of the light at the output of the interferometer.
Frequency domain OCT; this uses a swept-frequency narrow spectrum source and a stationary interferometer. The axial response is obtained as the Fourier transform of the time-varying intensity of the light at the output of the interferometer.
We shall use the expression “Fourier domain” to cover both spectral domain and frequency domain.
Time domain OCT (the original, and currently the most prevalent, type) is limited in acquisition speed by the need for mechanical depth scanning, and has relatively poor signal-to-noise performance.
Fourier domain OCT (spectral or frequency domain) enables more rapid capture of high-resolution images without sacrificing sensitivity. The time for each axial scan (“A-scan” in ultrasound scanning terminology) is critical in medical in-vivo applications because of the need for the patient to stay still for the time that it takes to build up successive A-scans into a cross-sectional image (“B-scan”).
However, time domain OCT has one significant advantage: it is easy to combine dynamic focal adjustment in step with the mechanical time-delay scan, giving the optimum spot size at the depth which is being probed. In contrast, Fourier domain OCT acquires information from the whole depth at the same time, so it is not possible to dynamically adjust focus for best lateral resolution.
There are three main difficulties in providing a practical arrangement of an OCT probe in which the conflicting optical and medical requirements are resolved.
Firstly, there are difficulties in obtaining an image which is suitably in focus over the depth of the (A scan) image.
Secondly, to provide a B-scan image it is necessary to scan laterally across the surface. Designs exist for endoscopic probes which incorporate a miniature scanning device in the probe shaft tip, for instance using electro-magnetic coils to move the end of an optical fiber. This approach has the disadvantage of placing moving parts, and the power to drive them, inside the patient's body, and may increase the difficulty of sterilizing the equipment.
Thirdly, it is desirable to be able to provide a normal, full field, endoscope viewing channel at the same time.
Through this specification we will refer to “optical”, “light” and such terms. It will be understood, however that such terms refer to radiation of infra-red, visible or ultra-violet wavelengths as appropriate.